1. Field of the Invention
The present invention relates to a rotating speed detecting apparatus. In particular, the present invention relates to a rotating speed detecting apparatus having a bearing sensor.
2. Description of the Related Art
A sensor for detecting rotation of a motor has been known in the past. The sensor outputs detection pulses indicating that the rotation of the motor is detected. For example, the sensor outputs thousands of detection pulses when the motor makes one rotation. A calculating unit connected to the sensor receives the detection pulses, and is able to calculate a revolution (rotating speed) of the motor based on the number of the received detection pulses. The calculated revolution is used for controlling a vehicle equipped with the motor.
FIG. 1 is a figure for explaining a conventional method of calculating the rotating speed. Output pulses (output pulse group) output from the sensor are shown in FIG. 1. According to the conventional technique, the revolution of the motor is calculated (estimated) by counting the number of the output pulses in a predetermined sampling period. The count value becomes larger, as the motor rotates at a higher speed. The count value becomes smaller, as the motor rotates at a lower speed. However, there is a problem with the conventional technique in that computation error of the calculated revolution becomes larger as the rotating speed of the motor becomes lower. For example, let us consider a case where the count value of the output pulses is 8. In this case, a certain revolution is calculated by the calculating unit. However, an actual revolution is different between in a case where the revolution is calculated immediately after the eighth output pulse is output and in a case where the revolution is calculated immediately before the ninth output pulse is output. Therefore, the calculated revolution may differ greatly from the actual revolution.
Moreover, accuracy of the calculation of revolution also depends to the number of pulses (referred to as a “pulse number” hereinafter) output during the motor makes one rotation. The pulse number with regard to a typical sensor is, for example, 1000 to 2000. However, the pulse number with regard to a “bearing sensor” consisting of ball bearings and a pulse sensor is at most 100 to 200 because of its structure. In a case where the revolution of the motor is the same, the typical sensor associated with the large pulse number outputs more detection pulses than the bearing sensor associated with the small pulse number, as shown in FIG. 1. Therefore, when the bearing sensor is used, the number of pulses which the calculating unit receives during a sampling period is relatively small and hence the error of the calculated revolution is increased. In particular, when the motor rotates at a low speed, the number of pulses which the calculating unit receives becomes extremely small and thus the error of the calculated revolution becomes extremely large. This is not preferable from the viewpoint of the stable control of the vehicle.
Meanwhile, the above-mentioned bearing sensor is favorable in terms of cost and size. For example, a battery forklift is often equipped with the bearing sensor having a small pulse number. In this case, the cost can be reduced, and a large working space for an operator of the battery forklift can be obtained. It is desired to improve stability of a vehicle equipped with the bearing sensor such as the forklift. It is particularly desired to improve the stability when the vehicle runs at a low-speed.
Japanese Laid-Open Patent Application (JP-A-Heisei, 10-38906) discloses a technique relating to the detection of the rotating speed. According to the technique disclosed in the patent document, a pulse width of a pulse signal is detected at a predetermined calculation timing on the basis of a middle edge calculating method. When a change in a pulse signal has been detected, the pulse width of the pulse signal is measured at the calculation timing. On the other hand, when a change in a pulse signal has not been detected, the pulse width of at the calculation timing is estimated on the basis of a difference between the latest pulse width and the previous pulse width.